(a) Technical Field
The present invention relates to a functional reinforcing filler including inorganic particles surface-modified with an alkenylsilanol obtained from hydrolysis of an alkenylalkoxysilane compound.
(b) Background Art
Since the mid-20th century, silanes having organic functional groups have been widely used to enhance adhesion between silica and polymers or to improve compatibility. But, it is reported that a silica-filled rubber composition does not have the desired reinforcing effect and an excellent improvement in physical properties is attained when a sulfur-containing coupling agent such as 3-mercaptopropyltrimethoxysilane (MPTMS) is used [U.S. Pat. No. 0,176,852 A1]. However, the mercaptoalkyltrialkoxysilane coupling agent has offensive odor and, when it is mixed with a polymer composition, processability is degraded since the prevulcanization time is greatly reduced due to the highly reactive thiol (—SH) group.
In the early 1970s, bis(alkoxysilylalkyl)polysulfides[(RO)3SiCH2CH2CH2SxCH2CH2CH2Si(OR)3] were developed [U.S. Pat. No. 3,842,111, U.S. Pat. No. 4,384,132, U.S. Pat. No. 4,507,490]. And, in the early 1990s, Michelin announced the “green tires” using bis(triethoxysilylpropyl)tetrasulfide (TESPT) [Eur. Patent EP 0501227, U.S. Pat. No. 5,227,425]. Since then, TESPT has been frequently used as filler along with silica in order to improve the physical properties of a rubber composition. However, TESPT is restricted in temperature when mixing with a rubber composition. For instance, when it is mixed with a rubber composition at high temperature, prevulcanization of the rubber mixture occurs because of irreversible thermal cracking of the polysulfane groups. And, when the mixing is performed at low temperature, the alkoxy group of TESPT may not be completely hydrolyzed. According to Wolff, S., complete hydrolysis of TESPT is difficult to be attained at low temperature because of steric hindrance [Wolff, S. Kautsch. Cummi, Kunstst 1981, 34, 280]. As a result, the residual alkoxy group exists in the molecule and hydrolysis occurs continuously even after mixing with the rubber composition. It decreases the life span of the rubber mixture as it is released as alcohol from inside the rubber matrix.
Thus, highly dispersible silica capable of minimizing the release of alcohol and enhancing the dispersibility of silica is consistently developed by silica manufacturers [U.S. Pat. No. 044,037]. However, use of TESPT cannot be free from the release of alcohol. At present, most of silica-containing rubbers use TESPT or MPTMS, and it is quite uncommon to use a silane having a sulfur-free organic functional group, which allows less improvement in rubber properties, in a rubber mixture for a tire.
It is known that when an alkenylalkoxysilane having a sulfur-free organic functional group is mixed with a rubber together with silica, rubber properties can be improved by using a catalyst such as butyllithium or peroxide [U.S. Pat. No. 0,019,554 A1]. However, in that case, coupling tends to occur with the silane coupling agent rather than the coupling between silica and rubber due to hydrolysis of the chlorine or alkoxy group of the silane, thus resulting in the formation of polysiloxane. This leads to insufficient improvement in the properties of the tire composite and inevitably leads to unfavorable processability and economic loss because of the need of additional use of additives in the manufacture of rubber for tires.
In contrast, alkenylsilane, which is commonly used in the manufacture of a rubber mixture, allows a long prevulcanization time when mixing with a rubber composition and the mixing temperature is not particularly restricted. However, liquid alkenylsilanes, especially vinylsilane and allylsilane, have storage problems because of fast hydrolysis. During mixing with the rubber mixture, they are hydrolyzed quickly, leading to fast condensation with the coupling agent rather than the silica-coupling agent-rubber coupling. Also, a larger amount of sulfur is required as compared to when a sulfur-containing coupling agent, e.g. MPTMS or TESPT, is used. That is to say, since the coupling agent is sulfur-free, solid sulfur or peroxide has to be added. Unless the rubber mixture is mixed homogeneously, the desired improvement in physical properties cannot be attained.
In general, silanetriol is synthesized by hydrolyzing chlorosilane or alkoxysilane. As a typical example of using chlorosilane, Jutzi et al. reported synthesis of (1-trimethylsilylcyclopenta-2,-4-dienyl)silanetriol with a yield of 98% by dissolving (1-trimethylsilylcyclopenta-2,-4-dienyl)trichlorosilane, a chlorosilane with large steric hindrance, in ethyl ether, slowly adding an aniline aqueous solution and stirring at 0° C. for 3 hours, removing the resulting aniline salt through filtration and then removing ethyl ether under reduced pressure [Organometallics 1997, 16, 5377]. However, this method involves a complicated process of removing a large amount of salts and is limited in that it is applicable only to silanes having substituents with large steric hindrance. As a typical example of using alkoxysilane, Ishida et al. obtained cyclohexylsilanetriol by mixing cyclohexyltrimethoxysilane in an aqueous solution of acetic acid and stirring for 2 hours at room temperature [J. Polym. Sci. 1979, 17, 1807]. In addition, Korkin et al. reported that they obtained phenylsilanetriol with a yield of 68% by adding phenyltrimethoxysilane dropwise to an acetic acid aqueous solution, stirring for 4 hours while maintaining temperature at 5-10° C. and removing impurities from the resulting white solid through filtration [J. Organomet. Chem. 2003, 686, 313]. However, the method of using alkoxysilane is limited in that the resultant silanetriol should be insoluble in water.